Fix "Uncaught Error: Call to undefined function wcs_get_subscriptions_for_order()"

[rvdsteege]

It appears a fatal error can occur if WooCommerce Subscriptions is not active:

CRITICAL Uncaught Error: Call to undefined function wcs_get_subscriptions_for_order() in /wp-content/plugins/pronamic-ideal/vendor/wp-pay-extensions/woocommerce/src/Gateway.php:792

wp-pronamic-pay-woocommerce/src/Gateway.php

Lines 784 to 810 in 7317828

	/**
	* Connection subscription payment renewal.
	*
	* @param Payment $payment Payment.
	* @param WC_Order $order WooCommerce order.
	* @return void
	*/
	private function connect_subscription_payment_renewal( $payment, $order ) {
	$woocommerce_subscriptions = \wcs_get_subscriptions_for_order( $order, [ 'order_type' => 'renewal' ] );
	foreach ( $woocommerce_subscriptions as $woocommerce_subscription ) {
	$subscription_helper = new SubscriptionHelper( $woocommerce_subscription );
	$pronamic_subscription = $subscription_helper->get_pronamic_subscription();
	if ( null !== $pronamic_subscription ) {
	// Add subscription and period.
	$payment->add_subscription( $pronamic_subscription );
	$period = $pronamic_subscription->next_period();
	if ( null !== $period ) {
	$payment->add_period( $period );
	}
	}
	}
	}